Active brazing solders are alloys which contain at least one element having an affinity for oxygen, such as titanium. They attack the covalent or ionic bonding of the ceramic surfaces to be brazed, wet these surfaces, and thus unite chemically and mechanically with them. Therefore, brazing requires no fluxes whatsoever.
Part of these active brazing solders, if they are brittle and difficult to machine or unmachinable in the solidified condition, can be produced by melt-spinning in the form of thin strips, which can then be easily machined, e.g., stamped or cut.
Thus, shaped active brazing foil parts, such as rings, can be produced, which are placed between the ceramic parts to be brazed and are subsequently fused with the latter.
Part of the molten and solidified active brazing alloys may also be ground into powder and processed in this form into an active brazing paste, which can also be introduced between the ceramic parts, e.g., in the form of a ring, and subsequently fused with these parts.
When brazing ceramic parts of alumina, particularly of 99.9 percent, i.e., high-purity, alumina as is needed and used for capacitive or resistive ceramic pressure sensors, particularly absolute-pressure sensors, the active brazing solder must meet several requirements; in particular, it must have the following properties:
The temperature at which the sintered alumina ceramic is brazed, i.e., the brazing temperature, must be below 1000.degree. C., preferably between 700.degree. C. and 980.degree. C. PA1 The brazed joint must be high-vacuum-tight over a long period of time, so that a vacuum existing during the brazing process in the chamber of a pressure sensor, for example, which is closed by the brazing, will remain unchanged. PA1 The coefficient of thermal expansion of the active brazing alloy should be identical to that of the alumina ceramic in the entire temperature range covered during the brazing process, so that only minimal stress will be developed during cooling from the brazing temperature to the ambient temperature. PA1 The strength of the brazed joint between the two ceramic parts must be so high that under tensile loading, fracture will result not at the joint, but in the adjacent ceramic. PA1 The pressure resistance of the active brazing solder must be at least 2 GPa (=2 Gigapascals). PA1 An active brazing solder which meets these requirements should also be processable into the aforementioned active brazing pastes, since the melt-spinning process, if applicable, requires costly and complicated equipment, so that the active brazing foils produced therewith are expensive.
With active brazing solders such as the zirconium-nickel-titanium alloys described in U.S. Pat. No. 5,351,938 (in the following abbreviated, as usual, as ZrNiTi alloys) not all of the above-mentioned boundary conditions can be fulfilled in a completely satisfactory manner. In particular, the above-mentioned requirement that the coefficients of thermal expansion of the active brazing solder and the alumina should be identical over the entire temperature range is not met, this requirement being based on new knowledge gained by the inventors.